I have used the DLink 2200 APs for about 6 months now with good Success. They have released firmware version 2.0 which is a signifigant improvement over version 1.0. You need to determine if the problem is the AP or the MAC. I am a PC guy so I can only offer PC based tools and tips. You can use Netstumbler to measure signal strength. It will also tell you which BSSID you are currently connected to. If you take your netstumbler laptop (or PDA) to the problem area, and identify the problem BSSID. Then you can log on to the individual APs and look at the config and the stats. You may have an AP with a failing radio. I hope your APs are alternating betwen channels 1, 6 and 11. You should pay attention to the signal strength and the connected speed when looking for problems. DLINK has a very complete spec sheet that shows the signal level required for each of the data rates. If your data rate is lower than the spec sheet says it should be then there may be noise or interference in the area on that channel. If clients are connecting several speed levels below the signal strength on the spec sheet you probably have a problem with that AP or that channel. This is all I can think of at this time.
If you really want to get detailed, get a sniffer trace over the air. Knoppix 3.9+ and the right wireless NIC card can provide you a free wireles sniffer using Tethereal.
I have a cluster of 7 DWL-2210AP Access Points setup throughout a school. The computers in use on campus are primarily Macs with OSX. The problem that happens is that the wireless network card on one of the macs will show full signal strength (4 [lets call them] "bars"). After a while (at MOST 1-3 weeks), once connected, it will stay connected for a few minutes MAX and then go down to 3 bars, then 2 bars, then 1 bars, then no more connection. At that point, I can disable and then re-enable the wireless network card on any of the macs and it will show full signal strength again for a few minutes MAX and then creep back down to no signal again. Eventually, the signal will not reconnect, even if I disable/enable the network card. At this point, I cold power-cycle the unit. This causes the unit to work for up to another 1-3 weeks and we start all over again.
I tried 3 different layouts of spreading them out throughout the school with no new results. The max overlap is 3 units (1 unit sees 2 other units, 3 units see 1 other unit, and the rest have no overlap within the same cluster) and that only occurs in 1 instance. Some of the units can see access points from the neighborhood, so I set the channel accordingly.
Any tips on further troubleshooting? Anyone know what I could be doing wrong? I have the SSID hidden, WEP enabled (some of the computers don't support WPA as of yet).